C. Delalande

Visible photoluminescence from porous silicon: A quantum confinement effect mainly due to holes?

We present results of photoluminescence experiments performed at 2 and 300 K on porous silicon layers with different porosities. The samples are obtained by electrochemical dissolution of (100) silicon wafers in hydrofluoric solution. The energy of the photoluminescence and its variation with porosity are found in good agreement with a theoretical model of quantum confinement in Si quantum wires. Electrons have to be taken into account, but it is shown that this quantum effect is mainly due (≊60%) to the large confinement of holes.

Unconventional motional narrowing in the optical spectrum of a semiconductor quantum dot

Abstract‘Motional narrowing’ refers to the striking phenomenon where the resonance line of a system coupled to a reservoir becomes narrower on increasing the reservoir fluctuation. A textbook example is found in nuclear magnetic resonance, where the fluctuating local magnetic fields created by randomly oriented nuclear spins are averaged when the motion of the nuclei is thermally activated. The existence of a motional-narrowing effect in the optical response of semiconductor quantum dots remains so far unexplored. This effect may be important in this instance because the decoherence dynamics is a central issue for the implementation of quantum information processing based on quantum dots. Here we report on the experimental evidence of motional narrowing in the optical spectrum of a semiconductor quantum dot broadened by the spectral-diffusion phenomenon. Surprisingly, motional narrowing is achieved when decreasing incident power or temperature, in contrast with the standard phenomenology observed for nuclear magnetic resonance.

Growth of Ga0.47In0.53As-InP quantum wells by low pressure metalorganic chemical vapor deposition

We describe the growth of multiquantum well and single quantum well Ga0.47In0.53As‐InP structures by low pressure metalorganic chemical vapor deposition. The multiwell structure consists of 25‐, 50‐, 100‐, and 200‐A quantum wells (Ga0.47In0.53As layers) separated by 500‐A barriers (InP layers). Auger measurements indicate the presence of four distinct wells with abrupt boundaries. Photoluminescence measurements are consistent with the existence of four wells; however, deviations are noted between experimentally determined and theoretically predicted recombination energies. An analogous situation exists for the single (50 and 100 A) quantum well structures. Possible explanations, including variation of well composition, variation of well thickness, and participation of impurities in the recombination process are suggested.

A semiclassical model for vibrational energy relaxation in simple liquids and compressed fluids

Using a simple model intermolecular potential, a density dependence of the vibrational population relaxation time in liquids is obtained which is in qualitative and quantitative agreement with experiment, over the whole range of density and temperature studied. This model is applicable to systems where the energy transfer involved is large with respect to thermal energies and describes the variation of the relaxation time as being essentially related to the variation of the collision frequency on the strongly repulsive part of the intermolecular potential. The formalism employed demonstrates clearly the role of excluded volume at high density and of bound states at low density and temperature.

Parametric oscillation in vertical triple microcavities

Optical parametric oscillation is a nonlinear process that enables coherent generation of ‘signal’ and ‘idler’ waves, shifted in frequency from the pump wave. Efficient parametric conversion is the paradigm for the generation of twin or entangled photons for quantum optics applications such as quantum cryptography, or for the generation of new frequencies in spectral domains not accessible by existing devices. Rapid development in the field of quantum information requires monolithic, alignment-free sources that enable efficient coupling into optical fibres and possibly electrical injection. During the past decade, much effort has been devoted to the development of integrated devices for quantum information and to the realization of all-semiconductor parametric oscillators. Nevertheless, at present optical parametric oscillators typically rely on nonlinear crystals placed into complex external cavities, and pumped by powerful external lasers. Long interaction lengths are typically required and the phase mismatch between the parametric waves propagating at different velocities results in poor parametric conversion efficiencies. Here we report the demonstration of parametric oscillation in a monolithic semiconductor triple microcavity with signal, pump and idler waves propagating along the vertical direction of the nanostructure. Alternatively, signal and idler beams can also be collected at finite angles, allowing the generation of entangled photon pairs. The pump threshold intensity is low enough to envisage the realization of an all-semiconductor electrically pumped micro-parametric oscillator.

Interferometric correlation spectroscopy in single quantum dots

We report high-resolution spectroscopy by interferometric correlation measurements on the photoluminescence signal of a single quantum dot. We demonstrate that the insertion of a Michelson interferometer in the detection path gives a compact and flexible setup for linewidth measurements. We have used this technique to study self-assembled InAs/GaAs quantum dots. We observe linewidth variations from one quantum dot to another, and we bring evidence of environment effects on the broadening processes.

Vibrational population relaxation of compressed nH2 fluid in the 15–110 K range

Experimental observations of the population relaxation time of the first excited vibrational level of nH2 have been made in the fluid state over a very wide range of temperature and pressure (15–110 K, 0–500 atm). These extensive measurements yield new insight into the fundamental parameters governing energy relaxation in simple fluids.

Vibrational energy relaxation in fluid mixtures: Hydrogen in argon

A recently developed model for vibrational population relaxation in pure fluids is extended to the case of mixtures. The predictions of the model are compared to experimental measurements in the hydrogen/argon system.

Third-order optical nonlinearities of carbon nanotubes in the femtosecond regime

Femtosecond pump–probe experiments have been carried out on an ensemble of single-wall carbon nanotubes deposited on a glass substrate. Measurements of transient changes of transmission and reflection provide an estimate of the real and imaginary parts of the second-order hyperpolarizability of carbon nanotubes. These values are compared with previous measurements and are discussed in the light of a simple model of the optical nonlinearities near the optical band-gap.

Optical studies of impurity trapping at the GaAlAs/GaAs interface in quantum well structures

We present low‐temperature photoluminescence studies of nominally undoped GaAs quantum wells in which weak acceptor‐related emissions can be observed. We investigate the influence of different sequences of prelayers, in which the number and the aluminum concentration of the layers are varied, on the impurity concentration in the quantum well, and show that thin layers of low aluminum percentage act as very efficient impurity trapping centers. We also present calculations of the electron to acceptor photoluminescence line shape, which show that the acceptor distribution has its maximum at the well interface, extending about 7 A in the barrier and 12 to 30 A in the well.